Method and apparatus for aluminum potline control

ABSTRACT

A method and apparatus for controlling the operation of aluminum reduction cells by controlling the position of the cell anode and by controlling the addition of alumina ore to the cell electrolyte. Anode position adjustment occurs after addition of alumina to the electrolyte and is based upon periodic measurements of resistance drop across the cell. Upon completion of anode position adjustment, the cell resistance is monitored by periodically determining first and second resistance values. The addition of alumina to the electrolyte is initiated upon detection of a predetermined difference between the current second resistance value and a base resistance value determined from the first resistance values.

nited States Patent [151 3,660,256 Lippitt et al. 1 May 2, 1972 METHOD AND APPARATUS FOR 3,471,390 10/1969 Kibby et al ..204/67 x ALUMINUM POTLINE CONTROL (diff/VI 16 681/57 Alt/00512 Primary Examiner-John H. Mack Assistant Examiner-D. R. Valentine Atlorney-William S. Wolfe, Gerald R. Woods, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [5 7] ABSTRACT A method and apparatus for controlling the operation of aluminum reduction cells by controlling the position of the cell anode and by controlling the addition of alumina ore to the cell electrolyte. Anode position adjustment occurs after addition of alumina to the electrolyte and is based upon periodic measurements of resistance drop across the cell. Upon completion of anode position adjustment, the cell resistance is monitored by periodically determining first and second re sistance values. The addition of alumina to the electrolyte is initiated upon detection of a predetermined difference between the current second resistance value and a base resistance value determined from the first resistance values.

5 Claims, 10 Drawing Figures PATENTEDMAY. 2 912 SHEET 01 [1F 10 mkv w NQNKQQQ A TTURNE Y PATENTEDPMY 2 1972 sum as or 10 m- MH PATENTEUMM 2I972 3.660256 saw 07 0F 10 snw 10 or 10 METHOD AND APPARATUS FOR ALUMINUM POTLINE CONTROL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the electrolytic process for aluminum reduction and, in particular, to an improved method and apparatus for controlling the operation of the aluminum reduction cells of a potline.

2. Description of the Prior Art The process for producing aluminum consists of essentially three steps: (a) the mining and purification of bauxite which is an ore mixture of minerals formed by the weathering of aluminum-bearing rocks; (b) the production of alumina ore (Al O from bauxite, normally by the Bayer process; and (c) the electrolytic reduction or smelting of alumina to obtain aluminum.

In the conventional reduction operation, alumina ore is dissolved in a molten salt electrolyte such as cryolite contained in a cell or pot having anode and cathode electrodes. Molten aluminum is deposited at the cathode by passing a very large current, in the order of thousands of arnperes, between the anode and cathode electrodes. A plurality of such cells are connected in series to form a potline.

Production of aluminum in a reduction cell is directly proportional to the electric current flowing through the cell and specifically to that part of the current that is utilized for the reduction of alumina ore to the metallic state. in turn, the amount of current that may be passed through a reduction cell for a given voltage is governed by the amount of electrical resistance obstructing the current flow. Resistance to the flow of current in an aluminum reduction cell consists of the following components: (a) the resistance of the current carriers; (b) the resistance of the molten electrolyte; and (c) the equivalent resistance of the specific electrochemical reaction. The voltage drop in the cell due to the specific electrochemical reaction in the cell is called the back EMF. Typically, the voltage between the anode and the cathode is in the order of 3.5 to 4 volts while the total voltage across a cell is in the order of 4.5 to 5 volts due to the resistance of the leads and various contact resistances.

The voltage associated with the electrochemical reaction thatoccurs in an aluminum reduction cell can be considered as an equivalentresistance. It is a function of the relative ion concentration as measured in terms of the percentage of alumina dissolvedin the molten electrolyte under particular conditions of cell temperature and current density. As the apparent alumina concentration in the electrolyte decreases below a particular level, a continuous gas film is rapidly formed on the anode surface, causing the resistance between anode and cathode to increase exponentially. This phenomenon is called anodeeffect and not only reduces production in the cell where it occurs but reduces production in the entire potline since the increased resistance reduces currentflow through the potline. The increase in cell resistance also causes generation of heat raising the temperature of the electrolyte and causing loss of fluoride from the electrolyte. Addition of alumina to the electrolyte causes the gas film on the anode to dissipate, returning the cell resistance to a normal value. While anode efiects are of primary concern in the operation of a potline, excess alumina in the cell electrolyte must also be avoided since excess alumina contaminates the metallic aluminum, and adversely affects the normal operation of the cell.

Sufficient heat must be generated in each reduction cell to offset the heat losses associated with the cell structure to thus maintain the cell within a desired temperature range. The heat that is generated is proportional to the product of the square of the current flowing through the cell and the resistance of the electrolyte and of the cell structure. The resistance of the electrolyte, in turn, is proportional to the product of its specific resistance and the anode-cathode distance divided by the effective cross-sectional area of the current path. Thus, for

a given electrolyte composition and area of current path, the resistance of the electrolyte is proportional to the distance between the anode and cathode of the reduction cell. Therefore, a balance must be maintained between the current passing through the cell and the anode-cathode spacing in order to maintain the cell within a desired temperature range, with resulting efiiciency of operation.

In controlling the operation of aluminum reduction cells, it is desirable to maintain the alumina concentration in the electrolyte within predetermined limits and to also control the distance between the anode and cathodeelectrodes of the reduction cell. Various automatic control arrangements for aluminum potlines have been proposed and implemented. in one such arrangement, control is based on measurement of the voltage between the anode and cathode electrodes of the reduction cell. In another arrangement, the rate of change of cell resistance is monitored and employed as the basic control parameter. In general, these methods of aluminum potline control have not been entirely successful because of continuous cycling or hunting and because of inability to reliably and accurately anticipate anode effects. Accordingly, it is desirable to provide an aluminum potline control system which provides effective and satisfactory control over the operation of aluminum reduction cells.

It is therefore an object of this invention to provide an improved arrangement for controlling the operation of an alu minum potline.

It is another object of this invention to provide a method and apparatus for more reliably controlling the reduction cells of an aluminum potline.

It is another object of this invention to provide a method and apparatus for reliably and accurately anticipating anode effects in aluminum reduction cells.

It is a further object of this invention to provide an improved method and apparatus for controlling the operation of aluminum reduction cells to improve aluminum potline efficiency.

SUMMARY OF THE INVENTION The foregoing objects are achieved, in accordance with one illustrated embodiment of the invention, by providing apparatus for sampling the current through the potline and the voltage drop across each reduction cell to calculate the average resistance of each cell over a 30-second interval. After addition of alumina to the electrolyte of a cell at the beginning of a control sequence, the 30-second resistance average is employed to control the positioning of the anode so that the resistance between the anode and cathode electrodes approximates a predetermined value. Upon completion of anode position adjustment, a resistance filter employs successive 30-second resistance averages to calculate a smoothed or filtered resistance value based on the most recent ISO-second resistance average and past filtered resistance and 30-second average resistance values. A resistance averaging unit is provided which utilizes the 30-second resistance averages to calculate the average resistance between the anode and cathode electrodes of the cell over successive five-minute periods.

The first output of the resistance averaging unit is employed as the base resistance value and stored in a base resistance storage unit. A first comparator is provided to compare the base resistance value with each successive 5-minute resistance average and causes the most recent S-minute resistance average to be stored in the base resistance storage unit if it is less than the current base resistance. A second comparator is provided to compare the base resistance value stored in the base resistance storage unit with the filtered resistance value and to provide an output signal when a predetermined difference is detected. The output signalof the second comparator causes addition of alumina to the cell electrolyte and initiates another control sequence. This sequence of anode positioning and resistance monitoring is repeatedly performed for each cell of the potline to detect the next imminent anode effeet in the cell and to add alumina to the cell to prevent the anode effect.

In a second embodiment of the invention, analog components are employed to perfonn similar functions to control the operation of aluminum reduction cells in a potline.

BRIEF DESCRIPTION OF THE DRAWINGS The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating an aluminum potline embodying and utilizing the present invention;

FIG. 2 is a diagram illustrating a multiplexed arrangement employing the control system of the invention;

FIG. 3 is a diagram graphically illustrating the relationship between the alumina concentration in the electrolyte of an alumina reduction cell and the resistance between the anode and cathode electrodes of the cell;

FIG. 4 is a block diagram illustrating one embodiment of the control system of the invention employed to control the operation of the aluminum potline of FIG. 1;

FIG. 5 is a timing diagram illustrating the operation of the sequence controller in the control system of FIG. 4,

FIG. 6 is a diagram graphically illustrating the operation of the control system of FIG. 4 in controlling aluminum potline operation;

FIG. 7 is a flow diagram illustrating operation of the control system of FIG. 4;

FIG. 8 is a block diagram illustrating another embodiment of the control system of the invention for controlling the operation of aluminum reduction cells; and

FIG. 9 is a timing diagram illustrating the operation of the sequence controller in the control system of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, two typical cells in a series of aluminum reduction pots, normally referred to as a potline, with associated apparatus are schematically illustrated. A typical potline normally comprises from 120 to 200 cells electrically connected in series. Electrical power is furnished to the potline by a suitable high-voltage DC supply, not shown in the drawing.

Each of the reduction cells or pots in the series, for example pots l0 and 11 in FIG. 1, comprise a vertically adjustable anode 12 formed of carbon, a cathode 13 serving as the crucible or container and also normally formed of carbon, an electrolytic bath 14, and a layer or pad of molten aluminum 15. The electrolyte 14 consists of an aluminum silicon fluorine salt called cryolite and fluorspar. A crust 16 of alumina is normally formed above the surface of electrolyte 14.

Anode 12 of each reduction cell is suspended in electrolyte l4 and supported by a steel rod or bar 20. The vertical position of bar 20 and hence the position of anode 12 relative to cathode l3 and aluminum pad is determined by an anode position actuator 21 which may be, for example, a reversible air motor with an associated gear mechanism for moving rod and anode l2 vertically up or down.

A current path from the high voltage supply through each reduction cell in the series is provided by bus bars 25 associated with each reduction cell. One end of each bus bar 25 is connected to the steel rod 20 supporting anode 12 of one reduction cell while the opposite end is connected to the cathode of the preceding reduction cell in the series. Electrical current in a reduction cell thus flows from the cathode of the preceding cell through bus bar 25, steel rod 20 and anode 12 of the cell and then through the electrolyte 14, aluminum pad 15, cathode l3 and bus bar 25 to the anode of the next cell in the series, as shown in FIG. 1. The magnitude of the current flowing through the series of reduction cells in the potline is measured by line current sensor associated with one of bus bars 25. The voltage drop across each reduction cell is measured by the cell voltage sensor 31 associated with each cell.

As the alumina in electrolyte 14 is reduced to aluminum in the reduction cell, the concentration of alumina in the electrolyte decreases. When the alumina concentration decreases below a particular level, the resistance of the cell increases, greatly increasing the voltage drop across the cell. This undesirable phenomenon is called "anode effect" and alleviation of this cell condition requires addition of alumina to electrolyte 14. Addition of alumina to electrolyte 14 of each cell is accomplished by knives 35 which are caused to move vertically downward by crust break actuator 36, striking crust 16 with sufficient force to break the crust. The broken crust is dissolved in electrolyte l4 and additional alumina ore is added to the cell through tube 38 by alumina feed actuator 39. A measured quantity of ore is provided to the cell by alumina feed actuator 39 to prevent excess concentration of alumina in the cell electrolyte. The added alumina ore forms a new crust above the surface of electrolyte l4. Crust break actuator 36 and alumina feed actuator 39 associated with each cell are normally pneumatic devices, as known in the art.

Control system 40, illustrated in FIG. 1, receives on line 41 the output signal of line current sensor 30, representing current flow through the series of reduction cells. Control system 40 also receives on lines 42 and 43 the output signals of cell voltage sensors 31 associated with cells 10 and 11 respectively representing the voltage drop across the cells. Control system 40 may receive the output signals of cell voltage sensors 31 on lines 42 and 43 and from the remainder of the cell in the potline in parallel, as shown in FIG. 1, or a multiplexing arrangement for transmission of these signals to control system 40, as known in the art, may be employed.

Control system 40 provides output signals on lines 44, 45 and 46 to anode position actuator 21, crust break actuator 36, and alumina feed actuator 39 respectively associated with reduction cell 10 to control the operation of cell 10. Control system 40 also provides output signals on lines 47, 48 and 49 to anode position actuator 21, crust break actuator 36, and alumina feed actuator 39 respectively associated with cell 11 to control the operation of cell 11. Control system 40 may transmit control signals to cells 10 and 11 and the remainder of the cells in the potline in parallel, as shown in FIG. 1, or a multiplexing arrangement for transmission of these signals to the cells, as known in the art, may be employed. In either a parallel or multiplexed arrangement, control system 40 may be shared by the cells of the potline, scanning the parameters of the cells, devoting a portion of its time to each cell and providing output control signals to each cell as required. Alternatively, separate control apparatus may be provided for control of each cell. The structure and functions of control system 40 are described in detail with reference to FIG. 4. A multiplexed arrangement is shown diagrammatically in FIG. 2.

In producing aluminum in an electrolytic reduction cell, current flowing through the cell reduces the alumina (A1 0 in solution in the cryolite. While there is no general agreement on the mechanism of alumina reduction, the following sequence of reactions is plausible. Alumina (A1 0 when dissolved in molten cryolite is ionized to form anions which are drawn by the potential gradient to the positively charged anode. At the anode the anion is discharged to form aluminum ions (AL +H-) and oxygen. The oxygen reacts with the carbon anode to form oxides of carbon. The aluminum ions (AI. -lH-) may either be attracted to the cathode by the potential gradient and reduced to form aluminum which is absorbed by the pad or may react with the cryolite to give (ALF 4 ions and sodium cations. At the cathode, sodium ions are discharged to give sodium which in turn reduces aluminum anions to give metallic aluminum. The carbon dioxide (CO formed at the anode is evolved as a gas with some carbon monoxide formed in the melt by the reduction of carbon dioxide (CO by entrained metallic aluminum. The quantity of molten aluminum in the cell increases as the process continues and periodically is removed by siphoning into a container. The carbon of the anodes is consumed during the process and the anodes are periodically replaced. At the normal temperature of operation of a reduction cell, the aluminum is slightly more dense than the molten electrolyte and thus covers the bottom .of the cell. For a detailed description of the operation of an aluminum reduction cell, see T.F.G. Pearson, The Chemical Background of the Aluminum Industry, Monograph No. 3, Royal Institute of Chemistry, I956.

As disassociation of the alumina into aluminum and oxygen progresses, the concentration of alumina in the electrolyte decreases. When the alumina concentration in the electrolyte is reduced toa certain level, the resistance of the reduction cell increases rapidly, causing the voltage across the cell to increase, within a few seconds, from a normal value of 4.5 to 5 volts to a value in the range of 30 to 50 volts. This anode eflect reduces the efiiciency-of the reduction process in the cell, i.e., reduces the amount of aluminum produced per ampere of current compared to the amount of production theoretically possible per ampere, in addition to reducing production in the entire potline since the increased resistance reduces current flow through the entire series of reduction cells. The anode effect in a cell also causes release of fluorine from the electrolyte, requiring addition of fluoride to the cell. It is therefore desirable in the operation of an aluminum potline to avoid anode effects in the reduction cells.

FIG. 3 illustrates graphically the relationship between alumina concentration in a reduction cell and the resistance between the anode and cathode electrodes of the cell. Referring to FIG. 3, as the alumina concentration in the electrolyte gradually decreases with time after addition of ore to the electrolyte at time t the resistance between the anode and cathode electrodes of the cell also initially decreases as indicated at A, gradually becoming substantially constant at B. When the alumina concentration in the electrolyte is reduced to a certain level indicated at C, the resistance between the anode and cathode electrodes of the reduction cell suddenly increases exponentially, as illustrated at D in FIG. 3 at approximately time The control method and apparatus of the invention enables anticipation of an anode effect, permitting continuous operation of the cells of an aluminum potline in the region of the resistance curve enclosed by the dashed line E in FIG. 3. The control method and apparatus of the invention thus permits more eflicient operation of a potline and the aluminum reduction cells of the potline by preventing the occurrence of anode effects in the cells.

The overall electrical energy emciency in reducing alumina to aluminum is in the range of 35 to 45 percent. The remainder of the electrical energy is converted to heat and maintains the reduction cell at a nominal operating temperature in the range of 950 to 970 C. It is desirable to maintain the cell within this temperature range since cell efficiency decreases as cell temperature increases above 970 C. Because the heat generated in a cell is directly proportional to the resistance of the electrolytic path between anode and cathode, it is therefore directly proportional to the distance between the anode and cathode and adjustment in anodecathode spacing may be made to maintain the cell within the system 40 shown in FIG. 1. Referring to FIG. 4, the signal on.

line 41 representing the current flow through the series of aluminum reduction cells is applied to resistance sampling unit 50. A signal representing the voltage across a selected reduction cell, in this case the signal on line 42 representing the voltage across cell 11 of FIG. 1, is also applied to resistance sampling unit 50. Resistance sampling unit 50 also receives signals from timing unit 51. In response to the timing signals provided by timing unit 51, resistance sampling unit 50 scans and samples the cell voltage signal on line 42 and the line current signal on line 41 every 5 seconds and stores six consecutive sampled values of cell voltage and line current. During the 30-second interval encompassing six samples of cell voltage and line current, resistance sampling unit 50 calculates a signal R representing the average resistance of the cell over the 30-second period. The function performed by resistance sampling unit 50 is described in the equation:

Where 1 R is the average resistance of the selected reduction cell over a specified period, for example 30 seconds.

V, represents the cell voltage sampled at intervals, for example every 5 seconds, during the specified period.

V, is the back EMF of the selected cell, i.e., the theoretical decomposition potential of alumina at the operating temperature of the cell, usually in the order of 1.5 volts.

1, is the current flowing through the cells of the potline as sampled at intervals, for example every 5 seconds, during the specified period.

Although a 30 second averaging period for resistance sampling unit 50 is specified for the illustrated embodiment of the invention, any suitable averaging period and any suitable sampling period and number of samples may be used. The function of resistance sampling unit 50 may conveniently be performed in a digital computer with the cell voltage values V, and the line current values I, being scanned and stored in the memory of the computer along with back EMF value V, and the computation being performed in the computer arithmetic unit. The timing function of timing unit 51 may accordingly be handled by the internal timing apparatus of the computer. Appropriate analog-to-digital conversion devices, as known in the art, may be employed to convert the analog current and voltage values to digital form foruse in the computer.

Output signal R of resistance sampling unit 50 representing the average resistance across a selected cell over an interval of 30 seconds is applied to resistance averaging unit 53, resistance filter 54 and anode position controller 55, as illustrated in FIG. 4. Resistance averaging unit 53 also receives appropriate timing signals from timing unit 51. The operation of resistance averaging unit 53 during a resistance monitoring period is initiated by a signal generated by sequence controller 65. Resistance averaging unit 53 calculates a resistance value R based on 10 consecutive outputs of resistance sampling unit 50 and represents the average resistance of the selected cell over a 5-minute period. The averaging function of resistance averaging unit 53 is described in the equation:

Where R is the average resistance of the selected reduction cell over a specified period, for example five minutes.

R represents consecutive outputs of resistance sampling unit 50 during the specified period. Although a 5-minute averaging period for resistance averaging unit 53 is specified for the illustrated embodiment of the invention, any convenient averaging period and number of outputs of resistance sampling unit 50 may be employed. The function of resistance averaging unit 53 may conveniently be performed in a digital computer with the consecutive 30- second resistance values R produced by resistance sampling unit 50 being stored in the computer memory and the calculation being performed in the arithmetic unit of the computer. The timing function of timing unit 51 and the control function of sequence controller 65 may be handled by the internal apparatus of the computer.

Output signal Ra) of resistance averaging unit 53 is supplied to base resistance storage unit 57 and to comparator 58. Base resistancestorage unit 57 stores the first output signal R of resistance averaging unit 53 during a resistance monitoring period, the stored resistance value being employed as the base resistance R, in the control system of the invention. Base resistance storage unit 57 may comprise a register or may be a memory storage location in a digital computer. The base resistance value R stored in unit 57 is applied to comparators 58 and 60.

Comparator 58 receives base resistance value R stored in base resistance storage unit 57 and successive output signals R of resistance averaging unit 53. Comparator 58 compares the magnitude of base resistance value R, with the magnitude of each -minute average resistance value R generated by resistance averaging unit 53. if the magnitude of base resistance R, is equal to or less than the magnitude of the current average resistance value R generated by resistance averaging unit 53, comparator 58 causes base resistance storage unit 57 to retain base resistance R However, if the magnitude of base resistance R is greater than the magnitude of the average resistance value R last generated by resistance averaging unit 53, as determined during comparison in comparator 58, comparator 58 causes the average resistance value R to be stored in base resistance storage unit 57 as new base resistance value R replacing the old value of base resistance. Thus, the first S-minute average resistance value R calculated by resistance averaging unit 53 during a resistance monitoring period is adopted as the base resistance in the control system of the invention and stored in base resistance storage unit 57. Each subsequent five-minute average resistance value R calculated by resistance averaging unit 53 is compared with the present base resistance in comparator 58 and is stored in base resistance storage unit 57 as the new base resistance value if it is less than the present base resistance value. Expressed mathematically, if

am Z b then the base resistance value in base resistance storage unit 57 remains unchanged. However, if a(0 b then R is stored in base resistance storage unit 57 as the new base resistance R.

Resistance filter 54 receives output signal R of resistance sampling unit in addition to timing information from timing 1 R m) IG-l) +goi aoa-n mml Where R is the new filtered value of cell resistance. R is the last filtered value of cell resistance calculated by resistance filter 54.

represents all earlier filtered values of cell resistance.

R is the last S-minute average of cell resistance calculated by resistance averaging unit 53.

represents all earlier 5-rninuto resistance average vzil IHS.

g and 81 are filter constants having typical values of 0.2 and 0.01 respectively. The values selected for g and g, are a function of the noise in the system and the reliability of the parameter measurements. The functions of resistance filter 54 may conveniently be performed in a digital computer with the various resistance and constant values being stored in the computer memory and the calculation being performed in the arithmetic unit. The calculation of filtered resistance R by resistance filter 54 is performed every time a new output signal R is provided by resistance sampling unit 50, viz. every 30 seconds in the illustrated embodiment. The functions of timing unit 51 in controlling the timing of resistance filter 54 may be conveniently performed by the internal timing apparatus of the computer. Output signal R of resistance filter 54 is applied to comparator 60.

Comparator receives output signal R of base resistance storage unit 57 in addition to filtered resistance value R from resistance filter 54. Comparator 60 compares the magnitudes of base resistance R,, and filtered resistance value R and generates output signal CRBR when filtered resistance value R exceeds base resistance value R,, by a predetermined amount, for example 0.3 micro-ohm. Comparator 60 may be arranged to generate signal CRBR when the difference between filtered resistance value R, and base resistance value R exceeds a selected value, typically in the range of 0.1 to 0.5 micro-ohm, depending upon the characteristics and operating conditions of the particular reduction cell.

Output signal CRBR of comparator 60 is applied to sequence controller 65. Sequence controller 65, as described in conjunction with resistance averaging unit 53 and resistance filter 54, controls the sequence of actions employed to maintain each reduction cell within desirable operating limits. In response to signal CRBR, sequence controller terminates the present control sequence and initiates a new control sequence, as described below. To this end, sequence controller 65 provides control signals to crust break controller 61, anode position controller 55, and alumina feed controller 67, as illustrated in FIG. 4.

The output of sequence controller 65 applied to crust break controller 61 causes controller 61 to energize crust break actuator 36. Actuator 36, in turn, breaks the crust above the surface of the electrolyte of the cell being monitored by the control system 40, adding alumina to the cell electrolyte. Crust break controller 61 also performs a counting function, registering and controlling the number of times knives 35 move vertically downward to break the crust of the associated cell.

The output of sequence controller 65 applied to anode position controller 55 actuates anode position controller 55 at a particular time during a control sequence. Anode position controller 55 also receives the signal R from resistance sampling unit 50 representing the 30-second average resistance value and an input from the operator specifying the desired anode-cathode resistance value to maintain proper cell temperature. Anode position controller 55 in turn causes operation of anode position actuator 21 to adjust the position of the cell anode relative to the cell cathode to attain the desired anode-cathode resistance.

Similarly, the output of sequence controller 65 applied to alumina feed controller 67 actuates controller 67 at a particular time during the control sequence. Alumina feed controller 67 in turn causes operation of alumina feed actuator 39 which adds alumina ore to the cell. Alumina feed controller 67 also perfomis a counting function, registering and controlling the number of times alumina feed actuator 39 dispenses ore to the associated cell. Sequence controller 65 also provides signals to resistance averaging unit 53 and to resistance filter 54 to initiate operation of unit 53 and filter 54 at a predetermined time during a control sequence. The functions of sequence controller 65, described in detail in the following paragraphs, may conveniently be performed by a digital computer.

In the operation of the control system of the invention, a control sequence for each reduction cell is divided into three phases:

a. crust break and alumina feed,

b. anode adjustment and stabilization,

c. cell resistance monitoring.

The end of a cell resistance monitoring phase and the beginning of a feed phase is indicated by the issuance of signal CRBR from comparator 60. In response to signal CRBR, sequence controller 65 provides a control signal to crust break controller 61, causing crust break controller 61 to initiate operation of crust break actuator 36, adding alumina to the cell electrolyte. After introducing an intervening delay of approximately 1 minute, sequence controller 65 provides an output signal to alumina feed controller 67 which initiates operation of alumina feed actuator 39, adding a predetermined quantity of new ore to the cell which forms a new crust.

After addition of alumina ore to the cell electrolyte, sequence controller 65 provides a signal to anode position controller 55 to initiate its operation. Anode position controller 55 receives output signal R of resistance sampling unit 50 and an operator input specifying the desired anodecathode resistance and utilizes this information to cause anode position actuator 21 to adjust the position of the cell anode relative to the cell cathode, if required, so that output signal R of resistance sampling unit 50 assumes a predetermined nominal value, for example 31.5 micro-ohms, as required for the particular cell to maintain its temperature in the desired range. Anode position adjustment may be necessitated by a variety of factors, for example deterioration of the carbon of the anode, a change in the temperature of the electrolyte or a change in the quantity of molten aluminum at the bottom of the cell.

In the illustrated embodiment, a period of 4 minutes is allotted for anode position adjustment and stabilization. Anode positioning is normally completed within 1 to 2 minutes after application of the control signal by sequence controller 65 to anode position controller 55, leaving a period of 2 or 3 minutes for stabilization of conditions within the cell. The cell resistance monitoring phase is next initiated by providing signals to resistance averaging unit 53 and resistance filter 54 which initiate their operation. The resistance monitoring phase continues for an indefinite period, typically between and 60 minutes, and terminates when comparator 60 detects a predetermined difference between filtered resistance value R and base resistance value R and generates signal CRBR, at which time sequence controller 65 interrupts the operation of resistance averaging unit 53 and resistance filter 54 and provides an output signal to alumina feed controller 67, as described in the preceding paragraph.

The operation of sequence controller 65 of the control system of FIG. 4 in controlling the operation of a cell is illustrated in FIG. 5. Referring to FIG. 5, in response to generation of signal CRBR by comparator 60, sequence controller 65 generates control signal SCSI which is applied to crust break controller 61 to cause crust breakage through the action of crust break actuator 36. After passage of a predetermined time, for example 1 minute in the illustrated embodiment of the invention, sequence controller 65 generates signal SCS2 which is applied to alumina feed controller 67 to cause addition of alumina ore to the cell by alumina feed actuator 39. Sequence controller 65 next generates control signal SCS3 which is applied to anode position controller 55 to initiate anode position adjustment through anode position actuator 21. A predetermined time is allotted for anode position adjustment and stabilization of cell conditions, for example four minutes in the illustrated embodiment of the invention, after which sequence controller 65 generates signal SCS4. Signal SCS4 is applied to resistance averaging unit 53 and resistance filter 54, initiating their operation to commence the resistance monitoring phase of the controller sequence. The control sequence is terminated by subsequent generation of the signal CRBR, initiating another control sequence for the cell.

FIG. 6 illustrates graphically typical variations of filtered cell resistance R, and base resistance R as alumina concentration in the electrolyte decreases, and exemplifies the operation of the control system of the invention. After crust break and alumina feed, the anode adjustment phase is entered.

Upon completion of the anode adjustment phase, sequence controller 65 initiates operation of resistance averaging unit 53 and resistance filter 54 to commence the cell resistance monitoring phase of the control sequence. The filtered cell resistance value R, initially decreases with the passage of time as alumina concentration in the electrolyte decreases. At the end of the first 5-minute averaging period, resistance averaging unit 53 calculates an average resistance value which is used as the base resistance R,,. Resistance averaging unit 53 calculates a new average resistance value at the end of each successive 5- minute period. As the 5-minute average resistance value decreases below the base resistance R it is employed as the base resistance value, as illustrated. The filtered resistance value and the 5-minute average resistance value eventually begin to increase. However, the base resistance value R, remains at the lowest 5-minute resistance value obtained up to that time during the resistance monitoring phase of the control sequence.

Assuming a disturbance in the cell which causes the resistance between the anode and cathode electrodes to decrease due to, for example, the effects of collapse of part of the cell crust, this resistance decrease is reflected in the decrease of filtered resistance value R, The average resistance calculated by resistance averaging unit 53 at the end of the 5- minute periodencompassing the disturbance will reflect this decrease in resistance. Comparator 58, in comparing the new 5-minute resistance average with the base resistance value stored in base resistance storage unit 57 will detect that the new 5-minute resistance average is less than the base resistance and will cause the new S-minute resistance average value to be stored in storage unit 57 as a new base resistance. This feature of the invention is illustrated in FIG. 6 in the reduction of the base resistance value at the end of the 5- minute period encompassing the disturbance.

As alumina concentration continues to decrease, the filtered and 5-minute cell resistance values again increase, as illustrated, while the base resistance value R remains at the lowest 5-minute resistance value. When comparator 60 detects a predetemiined difference, for example 0.3 microohms, between the filtered cell resistance value R, and the base resistance value R,,, as illustrated in FIG. 6, signal CRBR is generated, initiating the addition of alumina to the reduction cell to prevent the occurrence of an anode effect. As illustrated in the example of FIG. 6, the depletion of alumina in the cell electrolyte is detected at point A on the filtered resistance value curve. If the initial S-minute resistance value had been employed as the reference or datum, the requirement for additional alumina in the cell electrolyte would not have been detected until point B on the filtered resistance value curve. At point B, the probability of occurrence of an anode effect is much greater or an anode effect has already been initiated in the cell with resulting disturbance of the potline and reduction of efficiency.

The control system of the invention utilizing a moving base resistance which is responsive to changes over a period of time in the cell resistance thus pemiits prediction of an anode effect and corrective addition of alumina to the cell electrolyte before the anode efi'ect occurs. In the example of FIG. 5, adjustment of the base resistance value enabled earlier prediction, by approximately 2 or 3 minutes of the imminent anode effect permitting addition of alumina to the cell electrolyte to prevent the anode efiect and to thereby maintain the overall operating efiiciency of the cell and the entire potline. The control system of the invention thus provides adaptive control responsive to the operation and conditions in each cell of a potline to improve potline operation. Although the example of operation illustrated in FIG. 6 employs a difference value of 0.3 micro-ohm as the criterion for initiating addition of alumina to the cell electrolyte, the selection of this criterion depends upon the cell design and characteristics, the purity of the materials and operator practice and may range, for example, from 0.1 micro-ohm to 0.5 micro-ohm.

FIG. 7 illustrates a How chart of the operation of the control system of FIG. 4. Referring to FIG. 7, a control sequence for a selected cell directed by the control system of the invention is initiated by sequence controller 65 with the addition of alumina to the cell electrolyte. The anode position controller then determines, based on the 30-second resistance averages calculated by resistance sampling unit 50, whether or not the resistance of the selected cell approximates a predetermined desired resistance as set by the operator, i.e., is within a predetermined deadband" centered on the desired resistance. If not within the deadband, anode position controller 55 causes anode position actuator 21 to adjust the anode position until the anode-cathode resistance of the cell is within the deadband.

Upon completion of positioning of the anode to obtain a predetermined resistance between the anode and cathode electrodes of the cell and after a stabilization period has elapsed, monitoring of variations in anode-cathode resistance is initiated. Resistance filter 54 periodically calculates a filtered resistance value based on the present 30-second resistance average and all past filtered and 30-second average resistance values. The -minute average resistance values are calculated by resistance averaging unit 53 and the first such average is employed as the base resistance value. As each successive S-minute average resistance value is calculated, it is compared with the present base resistance value, and if less than the present base resistance value becomes the new base resistance value. As new filtered resistance values are calculated by resistance filter 54, each is compared with the base resistance value and if it exceeds the base resistance value by a predetermined amount, an imminent anode effect is indicated. The resistance monitoring phase is then terminated and alumina is added to the cell electrolyte to prevent occurrence of an anode effect, initiating a new control sequence.

In the description, the desirability of employing a digital computer to perform the functions of the control apparatus illustrated in FIG. 4 has been indicated. In such an implementation of the method and apparatus of the invention, analog-todigital and digital-to-analog converters would be employed to convert analog signals to digital quantities and digital representations to analog quantities, as required.

Alternatively, components as known in the art, for example analog components as described with reference to FIG. 8, may be employed to perform the described operations.

FIG. 8 illustrates the details of a second embodiment of the control system 40 shown in FIG. 1. Referring to FIG. 8, the signal on line 41 representing the current flow through the series of aluminum reduction cells is applied to Mv/I (millivolt/current) transmitter 80 which converts the voltage signal representing line current to a current signal for application to multiplier/divider 81. A signal representing the voltage across the selected reduction cell, for example the signal on line 42 representing the voltage across cell 11 of FIG. 1, is applied to magnetic amplifier 82 which serves to isolate the cell voltage sensor from the control system. The output voltage of magnetic amplifier 82 is applied to Mv/I transmitter 83 for conversion to a proportional current signal.

The output of Mv/l transmitter 83 is applied to proportional amplifier 84 along with a signal from loading station 85 representing the back EMF of the selected cell, i.e., the theoretical decomposition potential of alumina at the operating temperature of the cell, usually in the order of 1.5 volts. The output of proportional amplifier 84, representing the difference between the signals from Mv/I transmitter 83 and loading station 85, viz. cell voltageback EMF, is supplied to multiplier/divider 81. Multiplier/ divider 81 divides the output signal of proportional amplifier 84 by the output signal of Mv/l transmitter 80 as follows:

Where R is the output current signal of multiplier/divider 81 and represents the instantaneous resistance across the reduction cell. The resistance value R is normally represented by a current in the range of lO-SO milliamps.

Output signal R of multiplier/divider 81 is applied to resistance RC filter units 86 and 87. The resistance portions of units 86 and 87 convert the input current signal into corresponding voltage signals. The RC filter portion of unit 86 has a time constant of approximately 5 minutes while the RC filter portion of unit 87 has a time constant of approximately 30 seconds. Units 86 and 87 are connected to a reference voltage source through normally open contacts of relay R,-,, as illustrated in FIG. 8. The adjustable reference voltage source is employed to initialize units 86 and 87 to a standard value at the beginning of the resistance monitoring phase of a control sequence.

Output signal R5 of unit 86, representing the filtered value of cell voltage over a 5-minute period, is applied to Mv/I transmitter 88 which converts the signal to a proportional current signal for application to high/low selector 89. The output of high/low selector 89 is applied to a resistance 90 for conversion to a voltage signal, the output of resistance 90 being applied through the normally closed contacts of relay R to Mv/l transmitter 91. The current output signal of Mv/I transmitter 91 is applied to memory amplifier 92 along with timing pulses from timing unit 93. Memory amplifier 92 stores a base resistance value R the output signal of memory amplifier 92 being applied to high/low selector 89 and to proportional amplifier 94. High/low selector 89 provides an output to resistance 90 corresponding to the lowest of its two input signals R5 and R Memory amplifier 92 stores the output of high/low selector 89 in response to a timing pulse from timing unit 93. Timing unit 93 provides timing pulses at 5-minute intervals. Thus, every 5 minutes, base resistance R,, stored in memory amplifier 92 is compared with output signal R5 of resistance RC filter unit 86 and output signal R5 of unit 86 is then stored in memory amplifier 92 as the new base resistance if it is less than the present base resistance stored in amplifier 92. A circuit including a normally open contact of relay R is connected in parallel with Mv/l transmitter 88, high/low selector 89 and resistance 90, as illustrated to permit the standard value output of unit 86 to be stored in memory amplifier 92 at the beginning of the resistance monitoring phase of a control sequence. Timing unit 93 is connected to a suitable source of electrical energy through a normally closed contact of relay R Output signal R30 of unit 87 represents the filtered value of cell resistance over a 30-second interval and is applied to Mv/I transmitter 96 for conversion to a current signal. The output of Mv/l transmitter 96 is applied to controller 97 and to alarm units 98 and 102. The output signal of controller 97 is applied to integrator 99. A feedback path is provided from the output of integrator 99 to the input of controller 97. Controller 97 and integrator 99 produce a continuous filtering action analogous to that described by the equation for signal R discussed in conjunction with the FIG. 4 embodiment. Specifically, controller 97 includes a proportional path and an integrator. The effect of integrator 99 corresponds to the first term of the equation for R The effect of the controller proportional path corresponds to the second term of the equation R The effect of the controller integrator corresponds to the third term of the equation for R The net effect of controller 97 and integrator 99 is to provide a filtered output RF, Corresponding to output R of resistance filter 54 in the FIG. 4 embodiment. A circuit including a normally open contact of relay R;, is connected to controller 97 to adjust the charge on internal capacitors of controller 97 at the beginning of a resistance monitoring cycle so that signal RF is brought to equilibrium at a standard value.

Output signal RF of controller 97 and integrator 99 is applied to proportional amplifier 94 and to the input of controller 97. Proportional amplifier 94 functions to subtract base resistance value R, from filtered resistance value RF and rovides an output proportional to the difference to alarm unit 100. Alarm unit 100 is responsive to an output of amplifier 94 exceeding a predetermined value to generate sigial SSC. Signal SSC is applied through normally closed contacts of relay R to sequence controller 101 and serves to initiate operations in sequence controller 101.

Alarm unit 98 is responsive to output signals R30 of Mv/I transmitter 96 which exceed a predetermined value to also generate signal SSC and apply it through the normally closed contacts of relay R1 to sequence controller 101. Alarm unit 102 receives output signals R30 of Mv/I transmitter 96 and an operator set-point defining the desired cell resistance value. Alarm unit 102 provides an output to anode position controller 103 indicating to controller 103 if the resistance of the cell, as represented by signal R30 does not fall within a predetermined deadband centered on the operator setpoint value. The output of alarm unit 102 enables controller 103 to properly adjust the relative positions of the cell anode and cathode under control of sequence controller 101. Sequence controller 101 controls the sequence of actions employed to maintain the reduction cell within desirable operating limits, providing control signals to anode position controller 103, alumina feed controller 104 and crust break controller 105. Sequence controller 101 also provides signals to relays R R and R as required to direct the operation of the control system. Sequence controller 101 may be any suitable program-timer, for example a motor-driven drum with cam operated switches. The other components of the control system illustrated in FIG. 8, for example loading station 85, the Mv/l transmitters, the proportional amplifiers, multiplier/divider 81, high/low selector 89, memory amplifier 92, controller 97 and the alarm units are standard analog components known in the art. Such components are described in Process Control, Peter Harriott, McGraw Hill Publishing Company.

In operation, the control system embodiment of FIG. 8 cmploys a control sequence having three phases, viz. crust break and alumina feed, anode adjustment, and cell resistance monitoring, to control the operation of a reduction cell. During the course of the cell resistance monitoring phase, relays R and R are latched on and relay R is deenergized. Multiplier/divider 81 provides signal R representing the instantaneous value of cell resistance to units 86 and 87 which provide output signals R and R30 respectively. High/low selector 89 causes memory amplifier 92 to store as a base resistance value R the lowest value of signal R5 obtained during the cell resistance monitoring phase of the control sequence. Under the control of timing unit 93, the base resistance value R stored in memory amplifier 92 is updated, as required.

Controller 97 and integrator 98 receive output signal R30 of filter unit 87 to provide signal RF which is a function of all past values of signal RF Proportional amplifier 94 and alarm unit 100 provide output signal SSC if the difference between signals RF and R exceeds a predetermined value, for example 0.3 micro-ohm.

In response to signal SSC, sequence controller 101 provides control signal SCSA to crust break controller 105, causing crust break controller 105 to initiate operation of crust break actuator 36, adding alumina to the cell electrolyte. In addition, relays R, and R are latched off by signal SSC. After a delay of approximately 1 minute, sequence controller 101 provides control signal SCSB to alumina feed controller 104 to initiate operation of alumina feed actuator 39, adding a predetermined quantity of ore to the cell.

Upon completion of the crust break and alumina feed phase, sequence controller 101 provides control signal EFS to relay R latching relay R on. If alarm unit 98 determines from the value of signal R30 that the reduction cell is experiencing an anode effect, signal SSC is generated by alarm unit 98 and applied, through the normally closed contact of relay R to sequence controller 101 to cause sequence controller 101 to again issue control signal SCSA and SCSB, providing another crust break and alumina feed phase. The crust break and alumina feed phase is repeated until alarm unit 98 determines from the value of signal R30 that enough alumina has been added to the cell to terminate the anode effect.

Upon completion of the last crust break and alumina feed phase, sequence controller 101 provides control signal SCSC to anode position controller 103 to initiate its operation. Anode position controller 103 receives an output signal from alarm unit 102 which indicates whether or not the resistance across the cell approximates the operator setpoint value. It utilizes this information to cause anode position actuator 21 to adjust the position of the cell anode relative to the cell cathode, if required.

Upon completion of the anode adjustment phase of the control sequence, sequence controller 101 provides control signal SCSD to momentarily energize relay R and to latch relay R on. The energization of relay R causes application of a reference voltage to filter units 86 and 87, causing signals R5 and R30 to assume standard values greater than the normally anticipated values. This initial value of signal R5 is applied to memory amplifier 92 through the normally open contact of relay R and stored in memory amplifier 92 as the initial base resistance value R The initial value of R30 is applied to controller 97 and integrator 98 to bring output signal RF to equilibrium at a standard value at the beginning of a resistance monitoring phase. The energization of relay R also serves to adjust the charge on capacitors of controller 97 to enable controller 97 and integrator 98 to attain equilibrium at a standard value. Timing unit 93 is energized and provides a timing pulse to memory amplifier 92, enabling memory amplifier 92 to store the initial standard output value of filter unit 86 as the base resistance R,,.

FIG. 9 is a timing diagram illustrating the operation of the control system embodiment of FIG. 8. Referring to FIG. 9, in response to signal SSC from alarm unit 100, sequence controller 101 generates signals SCSA and SCSB to cause the crust break and alumina feed phase of the control sequence to be performed. At the end of this phase, signal EFS issues from sequence controller 101 to latch relay R on and enable an output from alarm unit 98 to be applied to sequence controller 101. Assuming that alarm unit 98 has not detected the existence of an anode effect, sequence controller 101 next provides control signal SCSC, causing performance of the anode adjustment and stabilization phase of the control sequence. Signal SCSD is next provided by sequence controller 101 to initialize the control system and to commence the resistance monitoring phase of the control sequence. A timing pulse is provided by timing unit 93 at this time to permit a standard value to be stored in memory amplifier 92 as the base resistance. Upon detection of a predetermined difference between the base resistance R stored in memory amplifier 92 and the output signal RF of controller 97 and integrator 99, signal SSC issues from alarm unit 100 to cause sequence controller 101 to terminate the resistance monitoring phase and to initiate the crust break and alumina feed phase of the next control sequence.

In the second control sequence shown in the timing diagram of FIG. 9, it is assumed that the conditions are such that the reduction cell actually experiences an anode efi'ect, in which event alarm unit 98 senses the existence of the anode effect at the end of the first crust break and alumina feed phase, issuing signal SSC to cause sequence controller 101 to initiate a second crust break and alumina feed phase prior to entering the anode adjustment and stabilization and cell resistance monitoring phases. A graphical illustration of the operation of the embodiment of FIG. 8 in controlling an aluminum reduction cell would be similar to the diagram of FIG. 6 applicable to the control system embodiment of FIG. 4.

Accordingly, there has been described herein a method and apparatus for aluminum potline control embodying the instant invention. All the principles of the invention have now been made clear in the illustrated embodiments, and there will be immediately obvious to those skilled in the art many modifications in structure, steps, arrangement, proportions, elements, materials, and components, used in the practice of the invention, and otherwise, which are particularly adapted for specific environments and operating requirements without departing from those principles. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

We claim:

1. A machine-implemented method for controlling the o eration of an alumina reduction cell having an anode, a cathode, and an electrolyte containing alumina in contact with the anode and cathode, said method including the reiterative steps of:

a. detennining a first resistance value based on the anode to cathode resistance of the cell during first periods;

b. determining a second resistance value based on the anode to cathode resistance of the cell during a second period, a second period being longer than a first period;

c. establishing a current base resistance value corresponding to the lowest one of the determined second resistance values;

d. comparing the current first resistance value and the current base resistance value; and

. adding alumina to the cell electrolyte when a predetermined difference between the compared values is detected.

2. A machine-implemented method for controlling the operation of an alumina reduction cell having an anode, a cathode and an electrolyte containing alumina in contact with the anode and cathode, said method including the reiterative steps of:

a. generating a first signal representing a filtered value of the anode to cathode resistance of the cell in a first period;

b. generating a second signal representing an average value of the anode to cathode resistance of the cell during a second period, the second period being longer than the first period;

c. generating a base signal equal to the lowest of all previously generated second signals;

d. comparing the current base signal with the current first signal; and

e. adding alumina to the electrolyte of the cell when the current first signal deviates from the current base signal by a predetermined amount.

3. A machine-implemented method for controlling the operation of an alumina reduction cell having an anode, a cathode and an electrolyte containing alumina in contact with the anode and cathode, said method including the reiterative steps of:

a. generating a first signal representing an average anode to cathode cell resistance during a first period;

b. generating a second signal representing an average anode to cathode resistance during a second period, a second period being longer than a first period;

c. generating a third signal during each of the first periods as a function of previously generated first signals and previously generated third signals;

d. adjusting the relative position of the anode and cathode until the first of the first signals approximates a predetermined value;

e. establishing a current base signal equal to the lowest of all of all previously generated second signals;

comparing the current base signal with the current third signal; and

g. adding alumina to the cell electrolyte when the current third signal exceeds the current base signal by a predetermined amount.

4. A machine-implemented method for controlling the a. generating a first signal at the end of each of successive first periods representing the average anode to cathode cell resistance durin the periodgenerating a secon signal at the end of each of successive second periods representing the average anode to cathode cell resistance during the period, each second period being longer than each first period;

c. generating a third signal at the end of each of the successive first periods as a function of previously generated first and third signals;

d. adjusting the relative position of the anode and cathode of the cell until the average resistance represented by the first signal at the end of a first period approximates a predetermined value;

. establishing upon completion of anode and cathode relative position adjustment a base resistance value corresponding to an average resistance value represented by the second signal at the end of a second period;

f. comparing at the end of each second period the average resistance value represented by the second signal with the base resistance value and replacing the base resistance value with the average resistance value when the average resistance value is the lower value;

g. comparing the current third signal with the current base resistance value at the end of each first period; and

h. adding alumina to the electrolyte of the cell when the current third signal exceeds the current first signal by a predetermined amount.

5. A. machine-implemented method as recited in claim 4 wherein the recited steps are repeated following each addition of alumina to the electrolyte of the cell.

t 1 l i t 

2. A machine-implemented method for controlling the operation of an alumina reduction cell having an anode, a cathode and an electrolyte containing alumina in contact with the anode and cathode, said method including the reiterative steps of: a. generating a first signal representing a filtered value of the anode to cathode resistance of the cell in a first period; b. generating a second signal representing an average value of the anode to cathode resistance of the cell during a second period, the second period being longer than the first period; c. generating a base signal equal to the lowest of all previously generated second signals; d. comparing the current base signal with the current first signal; and e. adding alumina to the electrolyte of the cell when the current first signal deviates from the current base signal by a predetermined amount.
 3. A machine-implemented method for controlling the operation of an alumina reduction cell having an anode, a cathode and an electrolyte containing alumina in contact with the anode and cathode, said method including the reiterative steps of: a. generating a first signal representing an average anode to cathode cell resistance during a first period; b. generating a second signal representing an average anode to cathode resistance during a second period, a second period being longer than a first period; c. generating a third signaL during each of the first periods as a function of previously generated first signals and previously generated third signals; d. adjusting the relative position of the anode and cathode until the first of the first signals approximates a predetermined value; e. establishing a current base signal equal to the lowest of all of all previously generated second signals; f. comparing the current base signal with the current third signal; and g. adding alumina to the cell electrolyte when the current third signal exceeds the current base signal by a predetermined amount.
 4. A machine-implemented method for controlling the operation of an alumina reduction cell having an anode, a cathode and an electrolyte containing alumina in contact with the anode and cathode, said method comprising the steps of: a. generating a first signal at the end of each of successive first periods representing the average anode to cathode cell resistance during the period; b. generating a second signal at the end of each of successive second periods representing the average anode to cathode cell resistance during the period, each second period being longer than each first period; c. generating a third signal at the end of each of the successive first periods as a function of previously generated first and third signals; d. adjusting the relative position of the anode and cathode of the cell until the average resistance represented by the first signal at the end of a first period approximates a predetermined value; e. establishing upon completion of anode and cathode relative position adjustment a base resistance value corresponding to an average resistance value represented by the second signal at the end of a second period; f. comparing at the end of each second period the average resistance value represented by the second signal with the base resistance value and replacing the base resistance value with the average resistance value when the average resistance value is the lower value; g. comparing the current third signal with the current base resistance value at the end of each first period; and h. adding alumina to the electrolyte of the cell when the current third signal exceeds the current first signal by a predetermined amount.
 5. A machine-implemented method as recited in claim 4 wherein the recited steps are repeated following each addition of alumina to the electrolyte of the cell. 